Coaxial transmission/center hub subassembly for a rotor assembly having ducted, coaxial counter-rotating rotors

ABSTRACT

One embodiment of a coaxial transmission/center hub subassembly for a rotor assembly having ducted, coaxial counter-rotating rotors includes a single stage transmission, a transmission housing, and a center hub support structure that are structurally and functionally interactive. The transmission housing includes upper and lower standpipe housings secured in combination with a middle housing. The single stage transmission includes an input pinion gear rotatably mounted in combination with the middle housing, and upper and lower spiral bevel gears rotatably coupled in combination with the input pinion gear to provide counter-rotation thereof. The spiral bevel gears include integral rotor shafts, respectively, rotatably mounted in the standpipe housings. The hub support structure is configured for securement of the middle housing internally in combination therewith, with respective surfaces thereof in abutting engagement so that dynamic loads of the rotors and thermal loads are directly coupled into the hub support structure via the middle housing while bending moments of the rotors are canceled in the middle housing. The hub support structure has radially extending arms for mounting the coaxial transmission/center hub subassembly in fixed coaxial relation to an airframe structure and for coupling the dynamic and thermal loads to the airframe structure. External surfaces of the standpipe housings function as sliding surfaces for linear motion of swashplate subassemblies to minimize the separation between the rotors so that the airframe structure has a compact aerodynamic and structural envelope. The transmission housing is internally configured to provide a splash lubrication subsystem.

RELATED APPLICATION

The present application is related to commonly-owned, U.S. Pat. No.5,150,857 entitled SHROUD GEOMETRY FOR UNMANNED AERIAL VEHICLES, tocommonly-owned, U.S. Pat. No. 5,152,478 entitled AN UNMANNED FLIGHTVEHICLE INCLUDING COUNTER ROTATING ROTORS POSITIONED WITHIN A TOROIDALSHROUD AND OPERABLE TO PROVIDE ALL REQUIRED VEHICLE FLIGHT CONTROLS, tocommonly-owned, co-pending U.S. patent application Ser. No. 07/903,061,filed 22 Jun. 1992, entitled A ROTOR BLADE SUBASSEMBLY FOR A ROTORASSEMBLY HAVING DUCTED, COAXIAL COUNTER-ROTATING ROTORS, tocommonly-owned, co-pending U.S. patent application Ser. No. 07/903,063,filed 22 Jun. 1992, entitled A SNUBBER ASSEMBLY FOR A ROTOR ASSEMBLYHAVING DUCTED, COAXIAL COUNTER-ROTATING ROTORS, to commonly-owned,co-pending U.S. patent application Ser. No. 07/903,064, filed 22 Jun.1992, entitled AN INTEGRATED SPLINE/CONE SEAT SUBASSEMBLY FOR A ROTORASSEMBLY HAVING DUCTED, COAXIAL COUNTER-ROTATING ROTORS, tocommonly-owned, U.S. Pat. No. 5,226,350 entitled A DRIVE TRAIN ASSEMBLYFOR A ROTOR ASSEMBLY HAVING DUCTED, COAXIAL COUNTERROTATING ROTORS, andto commonly-owned, copending U.S. patent application Ser. No.07/903,060, filed 22 Jun. 1992, entitled TOROIDAL AIRFRAME STRUCTURE FORSHROUDED ROTOR UNMANNED AERIAL VEHICLES.

TECHNICAL FIELD

The present invention relates to unmanned aerial vehicles (UAVs), andmore particularly, to an optimized coaxial transmission/center hubsubassembly for a rotor assembly for a UAV having a toroidal fuselage(shroud) and a pair of coaxial, counter-rotating, ducted, multi-bladedrotors.

BACKGROUND OF THE INVENTION

There has been a recent resurgence in the interest in unmanned aerialvehicles (UAVs) for performing a variety of missions where the use ofmanned flight vehicles is not deemed appropriate for whatever reason.Such missions include surveillance. reconnaissance, target acquisitionand/or designation, data acquisition, communications datalinking, decoy,jamming, harassment, or one-way supply flights. This interest hasfocused mainly on UAVs having the archetypical airplane configuration,i.e., a fuselage, wings having horizontally mounted engines fortranslational flight, and an empennage, as opposed to "rotor-type" UAVs,for several reasons.

First, the design, fabrication, and operation of "winged" UAVs is but anextrapolation of the manned vehicle flight art, and therefore, may beaccomplished in a relatively straightforward and cost effective manner.In particular, the aerodynamic characteristics of such UAVs are welldocumented such that the pilotage (flight operation) of such vehicles,whether by remote communications datalinking of commands to the UAVand/or software programming of an on-board flight computer, isrelatively simple.

In addition, the range and speed of such UAVs is generally superior torotor-type UAVs. Moreover, the weight-carrying capacity of such UAVs isgenerally greater than rotor-type UAVs such that winged UAVs may carry alarger mission payload and/or a larger fuel supply, thereby increasingthe vehicle's mission efficiency. These characteristics make winged UAVsmore suitable than rotor-type UAVs for certain mission profilesinvolving endurance, distance, and load capability. Winged UAVs,however, have one glaring deficiency that severely limits their utility.

More specifically, winged UAVs do not have a fixed spatial point"loiter" capability. For optimal performance of many of the typicalmission profiles described hereinabove, it is desirable that the UAVhave the capability to maintain a fixed spatial frame of reference withrespect to static ground points for extended periods of time, e.g.,target acquisition. One skilled in the art will appreciate that theflight characteristics of winged UAVs are such that winged UAVs cannotmaintain a fixed spatial frame of reference with respect to staticground points, i.e., loiter. Therefore, mission equipment for wingedUAVs must include complex, sensitive, and costly motion-compensatingmeans to suitably perform such mission profiles, i.e., maintenance of aconstant viewing azimuth for a static ground points.

Rotor-type UAVs, in contrast, are aerodynamically suited for suchloiter-type mission profiles. The rotors of the main rotor assembly ofsuch UAVs may be operated so that the UAV hovers at a fixed spatialframe of reference with respect to static ground points.

A need exists for rotary-type UAVs for a wide variety of reconnaissanceand/or communication missions, especially tactical reconnaissancemissions. Such UAVs may include a rotor assembly having ducted, coaxialcounter-rotating rotors. The rotor assembly should be design optimizedto provide a UAV airframe structure that is structurally andaerodynamically compact and lightweight. The rotor assembly should befurther design optimized to provide an optimal performance capability.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a coaxialtransmission/center hub subassembly for a rotor assembly having ducted,coaxial counter-rotating rotors that includes a single stagetransmission, a transmission housing, and a center hub support structurethat are structurally and functionally interactive.

Another object of the present invention is to provide a coaxialtransmission/center hub subassembly that includes a single stagetransmission that provides enhanced power transfer efficiency betweenthe engine and the counter-rotating rotors.

Yet another object of the present invention is to provide a single stagetransmission for the coaxial transmission/center hub subassembly thatincludes an input pinion gear rotatably mounted in combination with thetransmission housing and upper and lower bevel gears rotatably coupledin combination with the input pinion gear to provide counter-rotationthereof.

Still another object of the present invention is to provide a singlestage transmission for the coaxial transmission/center hub subassemblywherein the spiral bevel gears include integral rotor shafts rotatablymounted in the transmission housing.

A further object of the present is to provide a transmission housing forthe coaxial transmission/center hub subassembly that includes upper andlower standpipe housings secured in combination with a middle housing.

Yet a further object of the present invention is to provide a center hubsupport structure for the coaxial transmission/center hub subassemblythat is configured for securement of the middle housing internally incombination therewith, with respective surfaces thereof in abuttingengagement, so that dynamic loads of the counter-rotating rotors andthermal loads are directly coupled into the center hub support structurevia the middle housing and bending moments of the counter-rotatingrotors are canceled in the middle housing.

Still a further object of the present invention is to provide a centerhub support structure for the coaxial transmission/center hubsubassembly having radially extending arms for mounting the coaxialtransmission/center hub subassembly in fixed coaxial relation to anairframe structure and for coupling the dynamic and thermal loads to theairframe structure.

One more object of the present invention is to provide a coaxialtransmission/center hub subassembly wherein the external surfaces of theupper and lower standpipe housings function as sliding surfaces forlinear motion of rotor swashplate subassemblies to minimize separationbetween the counter-rotating rotors so that the airframe structure has acompact aerodynamic and structural envelope.

Still one more object of the present invention is to provide atransmission housing for the coaxial transmission/center hub subassemblythat is internally configured to provide a splash lubrication subsystem.

These and other objects are achieved by a coaxial transmission/centerhub subassembly according to the present invention that is designoptimized for unmanned aerial vehicles (UAVs) having a rotor assemblywith ducted, coaxial counter-rotating rotors. One embodiment of a UAVcomprises a toroidal fuselage or shroud having an aerodynamic profile,flight/mission equipment, a powerplant subsystem, and a rotor assembly.The toroidal fuselage is fabricated from a composite material as aclosed toroid to provide maximal structural strength. The toroidalfuselage has a plurality of support struts integrally formed with andextending radially outwardly from the inner periphery of the toroidalfuselage which are operative to support the rotor assembly in a fixedcoaxial relation with respect to the toroidal fuselage. The toroidalfuselage is configured to provide a plurality of accessible internalbays.

Forward located internal bays are typically utilized for sundryflight/mission equipment. Distribution of the various flight/missionequipment is optimized in conjunction with the placement to thepowerplant subsystem. The powerplant subsystem includes the fueltank(s), an engine, and a drive train assembly. The fuel tanks andengine are disposed within appropriate internal bays. The embodiment ofthe UAV described herein utilizes a Norton Motors rotary engine, ModelNR801T (modified as described hereinbelow), which provides a high powerto weight ratio and good partial power fuel consumption. The NR801Tengine is an air/liquid cooled engine that produces 45 HP at 6,000 RPM.Operation of the engine is controlled and monitored by the flightcomputer.

The standard Norton engine was modified by combining the functionalfeatures of the flywheel and the Plessey generator in an integratedflywheel/generator subassembly. The integrated flywheel/generatorsubassembly includes a large diameter, thin rotor having a plurality ofmagnets internally mounted therein, and a plurality of rigidly mountedstators. The drive train assembly of the described embodiment includes asprag clutch, an engine coupling subassembly, a drive shaft, and atransmission coupling subassembly. The drive train assembly is operativeto transfer the power developed by the engine to the rotor assembly.

The rotor assembly comprises an electronic control servo subsystem,upper and lower stationary swashplate subassemblies, a plurality ofpitch control rods, a coaxial transmission/center hub subassembly, upperand lower integrated spline/cone seat subassemblies, and upper and lowermulti-bladed, counter-rotating rotors integrated in combination with thetransmission/center hub subassembly. The rotors are aerodynamically"shrouded" by the toroidal fuselage. Blade pitch changes induced in thecounter-rotating rotors are utilized to generate all required lift,pitch, roll, and yaw control of the UAV. Such pitch changes are alsoutilized to regulate the pattern and velocity of airflow over thetoroidal shroud and into the rotor assembly. Such control of the airflowcreates a lifting component on the toroidal shroud that augments thelift provided by the counter-rotating rotors.

The electronic control servo subsystem is operative to control thefunctioning of the upper and lower stationary swashplate subassembliesby coupling inputs from the flight computer to the respective swashplatesubassemblies. The upper and lower stationary swashplate subassembliesare operative, in response to mechanical inputs from the linearactuators of the electronic control servo subsystem, to selectivelymechanically couple cyclic pitch inputs and/or collective pitch inputsto the respective counter-rotating rotors by means of the pitch controlrods, which are mechanically secured at the ends thereof to theswashplate subassemblies and rotor blade assemblies of thecounter-rotating rotors. The swashplate subassembly is design optimizedfor effective utilization in combination with the configuration of thecoaxial transmission/center hub subassembly.

The configuration of the transmission/center hub subassembly is designoptimized to provide an integrated, low component part system that islightweight, compact, and structurally and thermally efficient. Thetransmission/center hub subassembly includes a single stage transmissionsubsystem, a multi-member transmission housing, and a center hub supportstructure. The configuration of the transmission/center hub subassemblyprovides enhanced power transfer efficiency between the powerplantsubsystem and the counter-rotating rotors, thereby increasing theoperational capability and efficacy of the UAV. Further, thetransmission/center hub subassembly configuration minimizes theseparation between the upper and lower counter-rotating rotors, therebyproviding a UAV having a compact structural and aerodynamic envelope.The configuration of the transmission/center hub subassembly alsofacilitates the transfer of the dynamic loads developed by thecounter-rotating rotors, and reduces airframe vibration levels byproviding a direct load path between the upper and lowercounter-rotating rotors so that canceling of bending moments produced bythe rotors during flight operations occurs. In addition, thetransmission/center hub subassembly configuration according to thepresent invention eliminates the need for transmission mountingbrackets.

The single stage transmission subsystem comprises an input pinion gearhaving a splined end portion, bearings for mounting the input piniongear in rotatable combination with the transmission housing, and upperand lower spiral bevel gears. The upper and lower spiral bevel gearshave upper and lower rotor shafts, respectively, integrally formedtherewith, thereby eliminating the need for separate rotor shaftconnection means. The transmission subsystem further includes standpipebearings for rotatably mounting the respective upper and lower rotorshafts in combination with the transmission housing.

The input pinion gear is mechanically coupled the drive shaft by meansof the gear spline coupling (the gear spline coupling mechanicallyengages splined end portion of the pinion gear), and operative totransmit torque from the engine to the upper and lower spiral bevelgears. The placement the bevel gears vis-a-vis the spiral gear portionof the input pinion gear causes counter rotation of the upper and lowerrotor shafts with respect to one another. The splined end portion of theinput pinion gear facilitates quick disconnection of the single stagetransmission from the center hub support structure.

The multi-member transmission housing includes an upper standpipehousing, a lower standpipe housing, and a middle housing. The upper andlower standpipe housings are secured in combination with the middlehousing by means of screws. By mounting the upper and lower standpipehousings in combination with the middle housing, direct load paths areprovided for the dynamic and static longitudinal, lateral, vertical, andtorsional loads developed by the upper and lower counter-rotating rotorsinto the middle housing. This functional feature allows the operatingmoments of the upper and lower rotors to cancel each other out in themiddle housing. The cancellation function provided by the configurationof the transmission housing significantly reduces vibratory loads thatwould normally be transmitted to the toroidal fuselage.

The standpipe bearings are mounted against the internal surface of theupper and lower standpipe housings and are operative to facilitaterotary motion of the respective rotor shafts while transmitting rotorbending loads to the upper and lower standpipe housings. The respectivestandpipe bearings are separated to minimize shear reaction.

The coaxial transmission/center hub subassembly utilizes the externalsurfaces of the upper and lower standpipe housings as sliding surfacesfor the bidirectional translational movement of the respectivestationary swashplate subassemblies. The range of such bidirectionallinear motion is sufficient to couple the requisite collective pitchinputs to respective blades of the counter-rotating rotors for flightoperations of the UAV. By utilizing the external surfaces for swashplatemotion, a minimum separation between the upper and lowercounter-rotating rotors is achieved, thus providing the UAV with acompact structural and aerodynamic envelope.

The described embodiment of the coaxial transmission/center hubsubassembly includes a separate center hub support structure comprisinga cylindrical body having three equidistantly spaced integral supportarms extending radially outwardly therefrom. The support arms functionas the rigid attachment points for the support struts to mount thecoaxial transmission/center hub subassembly in fixed coaxial relation tothe toroidal fuselage.

The center hub support structure is configured so that the middlehousing may be slidably inserted therein such that external surfaces ofthe middle housing abuttingly engage internal surfaces of the center hubsupport structure. The abuttingly engaged surfaces in combinationfunction as mounting and load bearing surfaces that are operative totransfer the dynamic and static loads developed by the counter-rotatingrotors to the center hub support structure. The middle housing issecured in combination with the center hub support structure by means ofpins and screws. Dynamic lift loads developed by the counter-rotatingrotors are transmitted from the middle housing to the central hubsupport structure via the pins and screws. All other dynamic rotorloads, as well as thermal loads generated by operation of the singlestage transmission, are coupled from the middle housing to the centerhub support structure via the abuttingly engaged surfaces thereof.

The dynamic and static rotor loads and the thermal loads coupled intothe center hub support structure are transmitted into the toroidalfuselage, via the support struts, by means of the integral support arms.Cooling of the coaxial transmission/center hub subassembly, and inparticular the middle housing, is facilitated by the structuralarrangement wherein the center hub support structure, the support arms,and the support struts lie directly in the downwash generated by theupper rotor, thereby facilitating convective cooling of such structuralelements.

The cylindrical body further includes six mounting lugs extendingradially outwardly therefrom. The mounting lugs are utilized to mountthe electronic control servo subsystem (more specifically, the threelinear actuators thereof) in combination with the rotor assembly.

The coaxial transmission/center hub subassembly further includes asplash lubrication subsystem that provides oil lubrication for the inputpinion gear, the transmission bearings, the upper and lower spiral bevelgears, and the standpipe bearings. The upper standpipe bearing is greaselubricated due to its location vis-a-vis the splash lubricationsubsystem. The splash lubrication subsystem includes a pinion chamber,cored passages, and standpipe chambers formed in the upper and lowerstandpipe housings, respectively, which are fluidically interconnectedby means of a central reservoir.

The standpipe bearings, the transmission bearings, and the gear teeth ofthe input pinion gear, and the upper and lower spiral bevel gears areoil lubricated by means of the splash lubrication subsystem. Oil fromthe central reservoir is circulated, due to the rotary motion of theupper and lower spiral bevel gears, throughout the fluid flow pathwaysof the splash lubrication subsystem to lubricate the aforedescribedcomponents. Since no lubrication pumps are required for the splashlubrication subsystem, the overall system weight and complexity of theUAV is reduced.

The middle housing may be fabricated as an integral element of thecenter hub support structure to provide an alternative structuralembodiment of the coaxial transmission/center hub subassembly. Theintegrated center hub support structure provides the functions of themiddle housing in addition to the functions of the center hub supportstructure. In this embodiment, however, the securing pins and screws arenot required.

The upper and lower integrated spline/cone seat subassemblies areoperative to secure the upper and lower counter-rotating rotors,respectively, in combination with the coaxial transmission/center hubsubassembly. The integrated spline/cone seat subassembly is designoptimized to reduce the size/radial dimensions of the upper and lowerrotor shafts, the standpipe bearings, the standpipe housings, and theupper and lower stationary swashplate subassemblies. The downsizing ofthese components provides a significant savings in the overall systemweight of the UAV.

The integrated spline/cone seat subassembly includes a primary shaftportion having a first diameter, an end shaft portion having a seconddiameter, and a truncated conic transition portion intermediate theprimary, end shaft portions. Each end shaft portion has a plurality ofshaft spines extending radially outwardly therefrom. Eachcounter-rotating rotor includes a rotor hub that functions as part ofthe respective integrated spline/cone seat subassembly. Each rotor hubincludes a shaft aperture having a plurality of hub splines extendingradially inwardly from the wall defining the shaft aperture. The lowerportion of each hub spline has an outwardly tapered portion. The taperedportions of the hub splines abuttingly engage and are mechanicallysupported by the truncated conic transition portion of the respectiverotor shafts. The diameter of the primary shaft portion defines theradial dimensions of the respective rotor shafts, and, in consequence,the sizing of the standpipe bearings, the multi-member transmissionhousing, and the stationary swashplate subassemblies.

Each counter-rotating rotor includes the rotor hub, four snubberassemblies, and four rotor blade assemblies. The rotor hub additionallycomprises four outwardly extending arms having ends forming a clevis.Each clevis provides the means for securing the rotor blade assembly incombination with the rotor hub. The rotor hub also functions as anelement of the snubber assembly. Each outwardly extending arm is furtherconfigured to provide the means for securing the respective snubberassembly in combination with the rotor hub.

The snubber assembly comprises a spherical bearing, a bearing bolt, alocking nut, a snubber bracket secured in combination with the sphericalbearing, and securing bolts. The spherical bearing, snubber bracketcombination is rotatably mounted within the rotor hub by means of thebearing bolt, and secured in combination in the rotor hub by means ofthe locking nut.

Each counter-rotating rotor includes four rotor blade assemblies. Eachrotor blade assembly comprises an inner flexbeam, an integrated torquetube/spar member, an outer aerodynamic fairing or rotor blade, anoptimized blade joint and an optimized pitch control rod mountingscheme. Each rotor blade assembly has a tapered configuration thatprovides reduced weight, low inertia, a high chord frequency, animproved aerodynamic profile, low static droop, and eliminates highchordwise stresses and the need for blade damping mechanisms.

The flexbeam of the rotor blade assembly is a laminated compositestructure that is operative to react the centrifugal loads and amajority of the bending loads developed during operation of thecounter-rotating rotors. The flexbeam is secured in combination with therotor hub and the respective integrated torque tube/spar member andtapered rotor blade. To compensate for the variable strains inducedalong the span of the flexbeam, the flexbeam has a predetermined lineartwist, i.e., built-in twist, along the span thereof (inboard end tooutboard end). As a result of such pretwist, the pretwisted flexbeammakes an angle with respect to a horizontal plane that varies linearlyfrom about 0° at the inboard end (root section) of the pretwistedflexbeam to about 22° at the outboard end (tip section) of thepretwisted flexbeam.

The integrated torque tube/spar member of the rotor blade subassembly isformed as a continuous, single piece, low cost tubular compositestructure that provides high torsional and bending stiffness andfacilitates the utilization of the efficient blade joint. The integratedtorque tube/spar structure is formed as a continuous filament woundpiece that provides a continuous torsion load path and facilitates loadcoupling from the tapered rotor blade into the respective pretwistedflexbeam. The integrated torque tube/spar member includes an inboardtorque tube segment and an outboard spar segment. The spar segmentfunctions as the primary structural member of the rotor bladesubassembly and is operative to react all bending, torsional, shear, andcentrifugal dynamic loads developed during operation of thecounter-rotating rotors. The torque tube segment is operative to reactall torsional loads and some of the bending loads developed duringoperation of the counter-rotating rotors.

The configuration of the rotor blade of each rotor blade subassembly isdesign optimized for reduced weight utilizing composite materials, e.g.,high modulus graphite, which results in a tapered rotor blade having ahigh chordwise frequency. Each outboard segment of the rotor blades isconfigured to have an aerodynamic taper of about 2:1, which results in atapered rotor blade having a low outboard mass and a high inboardstiffness. The aerodynamic taper of the outer rotor blades results in alow moment of inertia about the hub centerline and the mass centroid ofeach tapered rotor blade being closer to the rotor hub. The highchordwise frequency of the tapered rotor blades provides the benefit ofrotor operation over a weaker modal response zone.

The optimized blade joint is operative to secure the pretwisted flexbeamin combination with the respective integrated torque tube/spar memberand tapered rotor blade. The blade joint includes an innovative boltlayout that is optimally positioned to eliminate the moment reaction atthe blade joint due to the steady chordwise loading experienced by therotor blade assembly, thereby allowing the utilization of smaller boltsand reduced joint thicknesses. The rotor blade subassembly furtherincludes an optimized pitch control rod mounting scheme that results inpressure forces acting on the strongest part of the control rod bearingsuch that longer effective lifetimes are achieved for the control rodbearings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the attendantfeatures and advantages thereof may be had by reference to the followingdetailed description of the invention when considered in conjunctionwith the accompanying drawings wherein:

FIG. 1 is a perspective, partially broken away view of one embodiment ofan unmanned aerial vehicle (UAV) according to the present invention.

FIG. 2 is a cross-sectional view illustrating a preferred aerodynamicprofile for the toroidal fuselage of the UAV of FIG. 1.

FIG. 3 is a cross-sectional view illustrating a drive train assembly forthe UAV according to the present invention.

FIG. 3A is an expanded cross-sectional view of one portion of the drivetrain assembly of FIG. 3.

FIG. 4 is a partial plan view illustrating one embodiment of a rotorassembly for the UAV according to the present invention.

FIG. 5A is a top plan view of one preferred embodiment of a swashplatesubassembly for the rotor assembly of FIG. 4.

FIG. 5B is a side plan view of the swashplate subassembly of FIG. 5A.

FIG. 6 is a cross-sectional view of one preferred embodiment of acoaxial transmission/center hub subassembly portion for the rotorassembly of FIG. 4.

FIG. 7 is a top plan view of the center hub support structure of thecoaxial transmission/center hub subassembly of FIG. 6.

FIG. 8 is a schematic representation of a prior art spline/cone seatarrangement for a rotor assembly.

FIG. 9 is a schematic representation of an integrated spline/cone seatsubassembly for the rotor assembly according to the present invention.

FIG. 9A is a top plan view of the rotor hub the integrated spline/coneseat subassembly according to the present invention.

FIG. 9B is a cross-sectional view of the rotor hub of FIG. 9A.

FIG. 10A is a top plan view of the top rotor assembly for the UAV of thepresent invention.

FIG. 10B is a partially broken away side plan view of the rotor assemblyof FIG. 10A.

FIG. 10C is a partial, enlarged view of FIG. 10B illustrating thesnubber assembly of the present invention.

FIG. 10D is a cross-sectional view of the snubber assembly taken alongline 10D--10D of FIG. 10C.

FIG. 11 is a graph depicting the operating curve for the UAV of thepresent invention vis-a-vis rotor assembly resonance mode conditions.

FIG. 12A is a cross-sectional view of the spar segment of the integratedtorque tube/spar member of the rotor blade assembly of the presentinvention.

FIG. 12B is a cross-sectional view of the torque tube segment of theintegrated torque tube/spar member of the rotor blade assembly of thepresent invention.

FIG. 13 is a graph defining the pretwist for the flexbeam of the rotorblade assembly according to the present invention.

FIG. 14 is a partial plan view depicting the optimal positioning of theblade joint of rotor blade assembly for the UAV of the presentinvention.

FIG. 15 is a graph showing the offset of the optimal blade jointposition with respect to the outboard mass centroid of the rotor bladeassembly of the present invention.

FIG. 16 is a schematic representation of a prior art pitch control rodbearing mounting scheme for a conventional rotor assembly.

FIG. 17 is a schematic representation of a pitch control rod bearingmounting scheme for the rotor assembly according to the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings wherein like reference numerals identifycorresponding or similar elements throughout the several views, FIGS. 1and 2 illustrate one embodiment of an unmanned aerial vehicle (UAV) 10according to the present invention. The illustrated embodiment of theUAV 10 comprises a toroidal fuselage or shroud 20 having an aerodynamicprofile 22, flight/mission equipment 30, a powerplant subsystem 50, anda rotor assembly 100. The aerodynamic profile 22 of the toroidalfuselage 20 of the described embodiment may be optimized to minimizenose-up pitching moments during forward translational flight. Onepreferred aerodynamic profile 22 for the illustrated UAV 10 is describedin further detail in commonly-owned, U.S. Pat. No. 5,150,857, entitled.SHROUD GEOMETRY FOR UNMANNED AERIAL VEHICLES, which is incorporatedherein by reference. Another embodiment of the UAV 10 according to thepresent invention, which includes a toroidal fuselage or shroud having ahemicylindrical aerodynamic profile, is described in commonly-owned,U.S. Pat. No. 5,152,478, entitled AN UNMANNED FLIGHT VEHICLE INCLUDINGCOUNTER ROTATING ROTORS POSITIONED WITHIN A TOROIDAL SHROUD AND OPERABLETO PROVIDE ALL REQUIRED VEHICLE FLIGHT CONTROLS. This embodimentutilizes cyclic pitch to compensate for the fuselage-induced nose-uppitching moments experienced during forward translational flight.

The embodiment of the UAV 10 described herein has a toroidal fuselage 20diameter of about 6.5 feet, a toroidal fuselage 20 envelope height ofabout 1.6 feet, an empty vehicle weight of about 175 pounds, and a grossvehicle weight of about 250 pounds. Reference numeral 12 illustrated inFIG. 1 defines the fuselage axis of the UAV 10. The toroidal fuselage 20has a plurality of support struts 24 (three for the describedembodiment) integrally formed with and extending radially outwardly fromthe inner periphery of the toroidal fuselage 20 to the rotor assembly100. The support struts 24, which are rigidly attached to the rotorassembly 100 as described hereinbelow in further detail, are operativeto support the rotor assembly 100 in a fixed coaxial relation withrespect to the toroidal fuselage 20, i.e., the rotational axis of therotor assembly 100 coincides with the fuselage axis 12. The supportstruts 24 are hollow structures to minimize the overall weight of theUAV 10, and to provide conduits for interconnecting operating elementsof the UAV 10. For example, the engine drive shaft (see descriptionhereinbelow) is routed through one of the support struts 24, asillustrated in FIG. 2. In addition, the electrical interface wiring forthe electronic control servo subsystem (see description hereinbelow) isrouted through another support strut 24.

The toroidal fuselage 20 and the plurality of support struts 24 arepreferably fabricated from composite material to provide a high strengthstructure of minimal weight. The various types of high tensile strengthfibrous materials and resins having utility in the formation ofaerospace composite structures are well known to those skilled in theart. The toroidal fuselage 20 is fabricated as a closed toroid toprovide maximal structural strength. The toroidal fuselage 20 is apartially hollow structure, and fabricated so as to provide a pluralityof accessible internal bays 26. One preferred embodiment of the toroidalfuselage 20 of the UAV 10 described herein is disclosed in furtherdetail in commonly-owned, co-pending U.S. patent application Ser. No.07/903,060, filed 22 Jun. 1992, entitled TOROIDAL AIRFRAME STRUCTURE FORSHROUDED ROTOR UNMANNED AERIAL VEHICLES, which is incorporated herein byreference.

Forward located internal bays 26 are typically utilized for sundryflight/mission equipment 30 as described hereinbelow. The missionpayload equipment 32 is preferably located, but not limited to, theinternal bay 26 at the 180° azimuthal station (the forward station).Generally, the mission payload equipment 32 will consist of some type ofpassive sensor(s), e.g., infrared detector(s), television camera(s),etc., and/or active device(s), e.g., laser(s), radio communicationsgear, radar, etc., and the associated processing equipment, and theforward internal bay 26 provides a good field-of-view for such missionpayload equipment 32. Other flight/mission equipment 30 such as avionics34, navigation equipment 36, flight computer 38, communications gear 40(for relaying real time sensor data and receiving real time commandinput signals), antennae, etc., are distributed in the various internalbays 26 as exemplarily illustrated in FIG. 1. Distribution of thevarious flight/mission equipment 30 is optimized in conjunction with theplacement of the powerplant subsystem 50 as described hereinbelow.

The powerplant subsystem 50 includes the fuel tank(s) 52, an engine 54,and a drive train assembly 60. The fuel tanks 52 are disposed withinappropriate internal bays 26, preferably in opposed equipment bays 26 atthe 90°, 270° azimuthal stations (the lateral stations) to maintain aconstant center of gravity for the UAV 10 during flight operations. Theengine 54 is mounted within an internal bay 26. The positioning of theengine 54 is selected to counterbalance the weight of the flight/missionequipment 30, which is disposed in the opposite portion of the toroidalfuselage 20 as described hereinabove. The embodiment of the UAV 10described herein utilizes a Norton Motors rotary engine, Model NR801T(modified as described hereinbelow), which provides a high power toweight ratio and good partial power fuel consumption. The NR801T engineis an air/liquid cooled engine that produces 45 HP at 6,000 RPM.Operation of the engine 54 is controlled and monitored by the flightcomputer 38.

The standard Norton engine described in the preceding paragraph wasdetermined to be deficient in several respects for utilization in theembodiment of the UAV 10 described herein. The standard Norton engineincludes a separate flywheel that is operative to store/release torqueenergy as required so that the Norton engine provides a relativelysteady torque output. The standard Norton engine further includes aseparate Plessey generator that is driven by the engine to provideelectrical power. The Plessey generator of the standard Norton engine isa heavy device having outsized dimensions.

As a result of these features and characteristics of the standard Nortonengine, the overall dimensional envelope of the standard Norton enginewas not compatible with the structural contours of the toroidal fuselagedescribed hereinabove (see also commonly-owned, co-pending U.S. patentapplication Ser. No. 07,903,060, filed 22 Jun. 1992, entitled TOROIDALAIRFRAME STRUCTURE FOR SHROUDED ROTOR UNMANNED AERIAL VEHICLES. Morespecifically, the standard Norton engine could not be mounted within theinternal bays 26 defined by the toroidal fuselage 20. Furthermore, theweight of the standard Norton engine would have significantly increasedthe overall gross weight of the UAV. The weight of the standard Nortonengine would have also resulted in an outboard shift in the center ofgravity of the UAV, which would have presented weight and balancedistribution problems with the flight/mission equipment 30.

The standard Norton engine was modified by combining the functionalfeatures of the flywheel and the Plessey generator in an integratedflywheel/generator subassembly 55 as illustrated in FIG. 3. Theintegrated flywheel/generator subassembly 55 is operative tostore/release torque energy as required so that the modified Nortonengine 54 provides a relatively steady torque output while concomitantlyproviding electrical power. The integrated flywheel/generatorsubassembly 55 includes a large diameter, thin rotor 56 having aplurality of magnets 57 internally mounted therein, and a plurality ofrigidly mounted stators 58. The rotor 56 is mechanically interconnectedto the bundt pan of the drive train assembly (described hereinbelow infurther detail) such that the modified Norton engine 54 provides thenecessary torque for rotation of the rotor 56.

The integrated flywheel/generator subassembly 55 weighs less than theseparate flywheel and Plessey generator of the standard Norton enginesuch that the overall gross weight of the UAV 10 is reduced. Inaddition, the dimensional envelope of the integrated flywheel/generatorsubassembly 55 permits the modified Norton engine 54 to be mountedwithin an internal bay 26 of the toroidal fuselage 20. Further, thedimensional envelope, relative positioning, and reduced weight of theintegrated flywheel/generator subassembly 55 result in an inboard shiftof the center of gravity of the modified Norton engine 54.

The drive train assembly 60 of the described embodiment of the UAV 10includes an over-running clutch such as a Borg-Warner sprag clutch.Over-running clutches are included as functional elements of the drivetrains of rotor assemblies to provide automatic decoupling between thedrive shaft and the engine when the engine is shut down. Such decouplingpermits the kinetic energy stored in the rotor assembly to beefficaciously and safely dissipated. Over-running clutches, however, donot function efficiently if subjected to over-hung loads, i.e., largeoperating moments and/or vibratory torque loads, such over-hung loadsinducing misalignments between the inner and outer races of the clutchhousings. To ensure effective functioning of over-running clutches,vibratory torque coupled into the over-running clutch and/or operatingmoments coupled through the over-running clutch bearings should beminimized.

The Norton engine 54 utilized in the described embodiment of the UAV 10produces a torque signature similar to a two-cycle internal combustionengine. Measurements taken during operation of such an engine revealedlarge torque irregularities of up to eight times the magnitude of thesteady state torque produced by the engine 54. Such torqueirregularities adversely affect the functional capabilities ofover-running clutches as described in the preceding paragraph.

Furthermore, the Norton engine 54 is mounted on soft shock absorbers(not illustrated) to attenuate engine loads and moments generated duringoperation of the engine 54. Operation of the engine 54 (as well as therotor assembly 100) can induce misalignments in the drive train driveshaft.

One preferred embodiment of the drive train assembly 60 for theembodiment of the UAV 10 described herein is illustrated in FIGS. 3, 3Aand includes a sprag clutch 62, an engine coupling subassembly 63, adrive shaft 72, and a transmission coupling subassembly 74. The drivetrain assembly 60 is operative to transfer the power developed by theengine 54 to the rotor assembly 100. The configuration of the drivetrain assembly 60 of the present invention is design optimized tomaximize the functional capability of the sprag clutch 62, i.e.,minimize/eliminate loads and/or moments that degrade clutch performance,and to accommodate maximum axial, angular, and parallel misalignmentbetween the engine 54 and the rotor assembly 100. In addition, theconfiguration of the drive train assembly 60 is operative to effectuatecancellation of loads developed by the integrated flywheel/generatorsubassembly 55 of the engine 54 described hereinabove.

With reference to FIGS. 3, 3A, the engine coupling subassembly 63includes a stud 64, a tapered adaptor or bundt pan 65, ball bearings 66,an external crown spline coupling 67, an internal spline coupling 68,and a pin-collar connector 69. The stud 64 provides a hard mount betweenthe engine coupling subassembly 63 and the tapered output shaft 54S ofthe engine 54. The stud 64 is mechanically interconnected to the bundtpan 65. The sprag clutch 62 is rigidly centered intermediate theexternal crown spline coupling 67 and the bundt pan 65 by means of theball bearings 66. The external crown spline coupling 67 is mechanicallyinterconnected (via complementary spline teeth 67T, 68T as illustratedin FIG. 3A) to the internal spline coupling 68.

One end of the drive shaft 72 is mechanically coupled to the enginecoupling subassembly 63 (more specifically the internal spline coupling68) by means of the pin-collar connector 69. The other end of the driveshaft 72 is mechanically coupled to the transmission couplingsubassembly 74 by means of a pin-collar connector 75. The transmissioncoupling subassembly 74 includes, in addition to the pin-collarconnector 75, an external crown spline coupling 76 and a gear splinecoupling 77. The external crown spline coupling 76 is mechanicallycoupled (via complementary spline teeth) to the gear spline coupling 77.The gear spline coupling 77 is configured for mechanical interconnectionwith the rotor assembly 100 as described in further detail hereinbelow.

The internal spline coupling 68 and the external crown spline coupling76 include additional material masses 70, 78 as illustrated in FIG. 3.These material masses 70, 78 are machined as required to facilitatebalancing of the drive shaft 72.

Torque from the engine 54 is transmitted to the engine couplingsubassembly 63 by means of the stud 64, tapered output shaft 54Scombination. The stud 64 couples torque to the bundt pan 65. Torque fromthe bundt pan 65 is coupled through the sprag clutch 62 to the externalcrown spline coupling 67, which in turn couples torque to the internalspline coupling 68 (via the complementary spline teeth 67T, 68T). Theinternal spline coupling 68 couples torque to the drive shaft 72, whichtransmits torque to the rotor assembly 100 via the transmission couplingsubassembly 74.

The drive shaft 72 of the drive train subassembly 60 is configured as atorque tube, having inside and outside diameters sized to providetorsional softness, i.e., the drive shaft 72 functions as a torsionalspring to isolate the coupling spline teeth 67T, 68T, the sprag clutch62, the transmission gearing (described hereinbelow in further detail),and the rotor assembly 100 from vibratory torque generated by theengine. The configuration of the drive shaft 72 eliminates the need forany additional torsionally soft couplings. The drive shaft 72 is notsupported by bearings, thereby reducing the installation weight of thedrive train subassembly 60. In addition, the configuration and couplingarrangements, i.e., internal spline coupling 68 and the external crownspline coupling 76, of the drive shaft 72 facilitate maximum axial,angular, and/or parallel misalignments between the rotor assembly 100and the engine 54 without degrading the functional capabilities thereof.

The mounting arrangement of the sprag clutch 62 and the external crownspline coupling 67 eliminates undesirable loads that could adverselyaffect performance of the sprag clutch 62. Since the external crownspline coupling 67 cannot react a moment, loads transmitted through theexternal crown spline coupling 67 react through the center of the spragclutch 62 such that misaligning moments that could degrade clutchperformance are not generated. Loads developed by the integratedflywheel/generator subassembly 55 are coupled into the bundt pan 65 andare effectively canceled in the bundt pan 65 adjacent the stud 64.

Preferably, the UAV 10 of the present invention includes an inlet screen14, disposed as partially illustrated in FIG. 1, to protect the rotorassembly 100 from FOD. The UAV 10 may also include an outlet screen (notillustrated) to similarly protect the rotor assembly 100.

One embodiment of the rotor assembly 100 of the present invention isillustrated in FIG. 4 and comprises an electronic control servosubsystem 102 that includes linear actuators 102LA, upper and lowerstationary swashplate subassemblies 80, a plurality of pitch controlrods 104, a coaxial transmission/center hub subassembly 110, upper andlower integrated spline/cone seat subassemblies 190, and upper and lowermulti-bladed, counter-rotating rotors 200, 202 integrated in combinationwith the transmission/center hub subassembly 110. The rotor assembly 100has a rotational axis 101 (see FIG. 6) that is coaxially aligned withthe fuselage axis 12. The rotors 200, 202 are aerodynamically "shrouded"by the toroidal fuselage 20. The rotors 200, 202 are preferably of therigid rotor type (as opposed to articulated rotors) to reduce thecomplexity and weight of the rotor assembly 100. Blade pitch changesinduced in the counter-rotating rotors 200, 202 are utilized to generateall required lift, pitch, roll, and yaw control of the UAV 10. Suchpitch changes are also utilized to regulate the pattern and velocity ofairflow over the toroidal shroud 20 and into the rotor assembly 100.Such control of the airflow creates a lifting component on the toroidalshroud 20 that augments the lift provided by the counter-rotating rotors200, 202. Additional structural and functional features of thecounter-rotating rotors 200, 202 are described in further detailhereinbelow.

The electronic control servo subsystem 102 is operative to control thefunctioning of the upper and lower stationary swashplate subassemblies80 by coupling inputs from the flight computer 38 of the UAV 10 to therespective swashplate subassemblies 80. The upper and lower stationaryswashplate subassemblies 80 are operative, in response to mechanicalinputs from the linear actuators 102LA A of the electronic control servosubsystem 102, to selectively mechanically couple cyclic pitch inputsand/or collective pitch inputs to the respective counter-rotating rotors200, 202 by means of the pitch control rods 104, which are mechanicallysecured at the ends thereof to the swashplate subassemblies 80 and rotorblade assemblies of counter-rotating rotors 200, 202, respectively.

An electronic control servo subsystem 102 especially designed for a UAVincorporating counter-rotating rotors is illustrated and described inU.S. Pat. No. 5,058,824, entitled SERVO CONTROL SYSTEM FOR A CO-AXIALROTARY WINGED AIRCRAFT, which is assigned to the assignee of the presentinvention, and which is incorporated herein by reference. Conventionalswashplate subassemblies, such as those described in the '824 patent,include a rotating swashplate and a stationary swashplate which areoperative in combination, through attitude or displacement changesinduced in the rotational plane of the rotating swashplate by thestationary swashplate, to provide pitch inputs to the blades of therotor assembly. In addition, in conventional in-line swashplatesubassemblies the rotating component thereof is located outboard withrespect to the stationary component of the swashplate subassembly.Further, the rotor blade--rotor hub attachment joint is outboard of thepitch control rod (which interconnects with the swashplate assembly).

A preferred embodiment of the stationary swashplate subassembly 80according to the present invention is illustrated in further detail inFIGS. 5A, 5B, and includes a central spherical ball bearing 82, astationary swashplate 83 of triangular configuration (the starmechanism) having three bearings 84 mounted in combination therewith, arotating swashplate 85 having four bearings 86 mounted in combinationtherewith, an annular bearing 87 intermediate the stationary androtating swashplates 83, 85 to facilitate rotary motion therebetween, arotating scissor hub plate 88, two rotating scissors 89 mechanicallyinterconnecting the rotating swashplate 85 and the rotating scissor hubplate 88, and two stationary scissors 90 mechanically interconnectingthe stationary swashplate 83 to respective stationary scissor supports91 (see FIG. 4) secured to the coaxial transmission/center hubsubassembly 110.

The stationary swashplate 83 is mounted in combination with the centralspherical ball 82 and operative for pivotal movement with respectthereto to provide cyclic pitch inputs to the multi-bladed,counter-rotating rotors 200, 202. Such pivotal motion is induced in thestationary swashplate 83 by means of the linear actuators 102LA (seeFIGS. 4, 5B) that are coupled to the stationary swashplate 83 by meansof the bearings 84. Pivotal motion of the stationary swashplate 83 withrespect to the central spherical ball 82 is facilitated by themechanical interaction between the stationary scissors 90 and therespective scissor supports 91.

Collective pitch inputs to the multi-bladed, counter-rotating rotors200, 202 are effectuated by bi-directional linear motion of thestationary swashplate 83, central spherical ball 82 combination inresponse to control inputs from the electronic control servo subsystem102 (via the linear actuators 102LA). Collective and cyclic pitch inputsare coupled from the stationary swashplate 83 to the rotating swashplate85. Such pitch inputs are coupled to the multi-bladed, counter-rotatingrotors 200, 202 by means of the pitch control rods 104, which aremechanically connected to the rotating swashplate 85 by means of thebearings 86. Mechanical coupling of the pitch control rods 104 to themulti-bladed, counter-rotating rotors 200, 202 is described in furtherdetail hereinbelow.

The swashplate subassembly 80 described hereinabove is design optimizedfor effective utilization in combination with the coaxialtransmission/center hub subassembly 110 as described in further detailhereinbelow. The swashplate subassembly 80 has an in-line configurationwherein the stationary point for pitch inputs, i.e., the bearings 84 ofthe stationary swashplate 83, is outboard of the rotating point, i.e.,the bearings 86 of the rotating swashplate 85, as illustrated in FIG.5B. The configuration of the swashplate subassembly 80 facilitatesmounting of the pitch control rods 104 in combination with themulti-bladed, counter-rotating rotors 200, 202, as described hereinbelowin further detail, approximately in-line with the respective snubberassemblies, thereby providing a rotor assembly 100 having a Delta 3 ofapproximately zero.

One embodiment of the coaxial transmission/center hub subassembly 110 isillustrated in further detail in FIGS. 4, 6-7. The configuration of thetransmission/center hub subassembly 110 is design optimized to providean integrated, low component part system that is lightweight, compact,and structurally and thermally efficient. The transmission/center hubsubassembly 110 includes a single stage transmission subsystem 120, amulti-member transmission housing 140, and a center hub supportstructure 160. The configuration of the transmission/center hubsubassembly 110 provides enhanced power transfer efficiency between thepowerplant subsystem 50 and the counter-rotating rotors 200, 202,thereby increasing the operational capability and efficacy of the UAV10.

Further, the transmission/center hub subassembly 110 configurationminimizes the separation between the upper and lower counter-rotatingrotors 200, 202, thereby providing a UAV 10 having a compact structuraland aerodynamic envelope. The configuration of the transmission/centerhub subassembly 110 also facilitates the transfer of the dynamic loadsdeveloped by the counter-rotating rotors 200, 202, and reduces airframevibration levels by providing a direct load path between the upper andlower counter-rotating rotors 200, 202 so that canceling of bendingmoments produced by the rotors 200, 202 during flight operations occurs.In addition, the transmission/center hub subassembly 110 configurationaccording to the present invention eliminates the need for transmissionmounting brackets.

With reference to FIG. 6, the single stage transmission subsystem 120comprises an input pinion gear 122 having a splined end portion 124,bearings 126 for mounting the input pinion gear 122 in rotatablecombination with the transmission housing 140, and upper and lowerspiral bevel gears 128, 130. The upper and lower spiral bevel gears 128,130 have upper and lower rotor shafts 128R, 130R, respectively,integrally formed therewith, thereby eliminating the need for separaterotor shaft connection means. The transmission subsystem 120 furtherincludes standpipe bearings 132, 134, 136, 138 for rotatably mountingthe respective upper and lower rotor shafts 128R, 130R in combinationwith the transmission housing 140. The integrated spline/cone seatsubassembly 190 (upper and lower) for securing the upper and lowermulti-bladed rotors 200, 202 in combination with respective rotor shafts128R, 130R (see FIG. 4) is described in further detail hereinbelow.

The input pinion gear 122 is mechanically coupled to the drive shaft 72by means of the gear spline coupling 77 (the gear spline coupling 77mechanically engages the splined end portion 124 of the pinion gear122), and operative to transmit torque from the engine 54 to the upperand lower spiral bevel gears 128, 130. The placement of the bevel gears128, 130 vis-a-vis the spiral gear portion of the input pinion gear 122causes counter rotation of the upper and lower rotor shafts 128R, 130Rwith respect to one another. The splined end portion 124 of the inputpinion gear 122 facilitates quick disconnection of the single stagetransmission.120 from the center hub support structure 160.

The multi-member transmission housing 140 includes an upper standpipehousing 142, a lower standpipe housing 144, and a middle housing 146.The upper and lower standpipe housings 142, 144 are secured incombination with the middle housing 146 by means of screws 148 (twelvein number). By mounting the upper and lower standpipe housings 142, 144in combination with the middle housing 146, direct load paths areprovided for the dynamic and static longitudinal, lateral, vertical, andtorsional loads developed by the upper and lower counter-rotating rotors200, 202 into the middle housing 146. This functional feature allows theoperating moments of the upper and lower rotors 200, 202 to cancel eachother out in the middle housing 146. The cancellation function providedby the configuration of the transmission housing 140 describedhereinabove significantly reduces vibratory loads that would normally betransmitted to the toroidal fuselage 20.

The standpipe bearings 132, 134 and 136, 138 are mounted against theinternal surface of the upper and lower standpipe housings 142, 144, asillustrated in FIG. 6. The standpipe bearings 132, 134, 136, 138 areoperative to facilitate rotary motion of the respective rotor shafts128R, 130R while transmitting rotor bending loads to the multi-membertransmission housing 140, i.e., the upper and lower standpipe housings142, 144. The respective standpipe bearings 132, 134, and 136, 138 areseparated to minimize shear reaction.

The coaxial transmission/center hub subassembly 110 of the presentinvention utilizes the external surfaces 142E, 144E of the upper andlower standpipe housings 142, 144 as sliding surfaces for thebidirectional translational movement of the respective stationaryswashplate subassemblies 80, as indicated by reference numerals 150, 152in FIG. 4. The range of such bidirectional linear motion is sufficientto couple the requisite collective pitch inputs to respective blades ofthe counter-rotating rotors 200, 202 for flight operations of the UAV10. By utilizing the external surfaces 142E, 144E for swashplate motion,a minimum separation between the upper and lower counter-rotating rotors200, 202 is achieved, thus providing the UAV 10 with a compactstructural and aerodynamic envelope.

The embodiment of the coaxial transmission/center hub subassembly 110illustrated in FIGS. 4, 6-7 includes a separate center hub supportstructure 160. With reference to FIG. 7, the center hub supportstructure 160 comprises a cylindrical body 162 having threeequidistantly spaced integral support arms 164 extending radiallyoutwardly therefrom. The support arms 164 function as the rigidattachment points for the support struts 24 to mount the coaxialtransmission/center hub subassembly 110 in fixed coaxial relation to thetoroidal fuselage 20.

The center hub support structure 160 is configured so that the middlehousing 146 of the multi-member transmission housing 140 may be slidablyinserted therein such that external surfaces 146E of the middle housing146 abuttingly engage internal surfaces 162I of the center hub supportstructure 160. The abuttingly engaged surfaces 146E, 162I in combinationfunction as mounting and load bearing surfaces that are operative totransfer the dynamic and static loads developed by the counter-rotatingrotors 200, 202 to the center hub support structure 160. The middlehousing 146 is secured in combination with the center hub supportstructure 160 by means of pins 168 (eighteen total) and screws 170 (sixtotal) as exemplarily illustrated in FIG. 6. Dynamic lift loadsdeveloped by the counter-rotating rotors 200, 202 are transmitted fromthe middle housing 146 to the central hub support structure 160 via thepins 168 and screws 170. All other dynamic rotor loads, as well asthermal loads generated by operation of the single stage transmission120, are coupled from the middle housing 146 to the center hub supportstructure 160 via the abuttingly engaged surfaces 146E, 162I thereof.

The dynamic and static rotor loads and the thermal loads coupled intothe center hub support structure 160 are transmitted into the toroidalfuselage 20, via the support struts 24, by means of the integral supportarms 164. Cooling of the coaxial transmission/center hub subassembly110, and in particular the middle housing 146, is facilitated by thestructural arrangement described hereinabove wherein the center hubsupport structure 160, the support arms 164, and the support struts 24lie directly in the downwash generated by the upper rotor 200, therebyfacilitating convective cooling of such structural elements.

The cylindrical body 162 further includes six mounting lugs 166extending radially outwardly therefrom as illustrated in FIG. 7. Themounting lugs 166 are utilized to mount the electronic control servosubsystem 102 (more specifically, the three linear actuators 102LAthereof) in combination with the rotor assembly 100.

The coaxial transmission/center hub subassembly 110 further includes asplash lubrication subsystem 174 that provides oil lubrication for theinput pinion gear 122, the transmission bearings 126, the upper andlower spiral bevel gears 128, 130, and the standpipe bearings 134, 136,138. The upper standpipe bearing 132 is grease lubricated due to itslocation vis-a-vis the splash lubrication subsystem 174. The splashlubrication subsystem 174 includes a pinion chamber 176, cored passages178, 180, and standpipe chambers 182, 184 formed in the upper and lowerstandpipe housings 142, 144, respectively, which are fluidicallyinterconnected by means of a central reservoir 186. Access to thecentral reservoir 186 is provided by means of an oil plug 188. Referencenumeral 189 represents the oil fill line for the central reservoir 186.

The standpipe bearings 134, 136, 138, the transmission bearings 126, andthe gear teeth of the input pinion gear 122 and the upper and lowerspiral bevel gears 128, 130 are oil lubricated by means of the splashlubrication subsystem 174. Oil from the central reservoir 186 iscirculated, due to the rotary motion of the upper and lower spiral bevelgears 128, 130, throughout the fluid flow pathways of the splashlubrication subsystem 174 as described in the preceding paragraph tolubricate the aforedescribed components. Since no lubrication pumps arerequired for the splash lubrication subsystem 174 described hereinabove,the overall system weight and complexity of the UAV 10 is reduced.

The middle housing 146 may be fabricated as an integral element of thecenter hub support structure 160 to provide an alternative structuralembodiment of the coaxial transmission/center hub subassembly 110described hereinabove. The integrated center hub support structureprovides the functions of the middle housing in addition to thefunctions of the center hub support structure. In this embodiment,however, the securing pins 168 and screws 170 described hereinabove arenot required.

The upper and lower integrated spline/cone seat subassemblies 190 of therotor assembly 100 are operative to secure the upper and lowercounter-rotating rotors 200, 202, respectively, in combination with thecoaxial transmission/center hub subassembly 110 as illustrated generallyin FIG. 4. The integrated spline/cone seat subassembly 190 of thepresent invention is design optimized to reduce the size/radialdimensions of the upper and lower rotor shafts 128R, 130R, the standpipebearings 132, 134, 136, 138, the standpipe housings 142, 144, and theupper and lower stationary swashplate subassemblies 80 describedhereinabove. The downsizing of these components provides a significantsavings in the overall system weight of the UAV 10 according to thepresent invention.

Traditional rotary aircraft mount the rotor hub in combination with therotor shaft by means of a spline and cone seat arrangement wherein thetwo elements are separate and distinct. With reference to FIG. 8, theconventional spline/cone seat arrangement comprises a rotor hub RHhaving a shaft aperture SA that includes a plurality of spaced apart hubsplines HS extending inwardly from the wall thereof, and a countersinkCK contiguous with the wall of the shaft aperture SA. The rotor shaft RSincludes a complementary plurality of shaft splines SS and acomplementary cone seat CS. The rotor hub RH slides downwardly onto therotor shaft RS so that the hub splines HS are interleaved with the shaftsplines SS and the countersink CK abuttingly engages the complementarycone seat CS.

The interleaved hub and shaft splines HS, SS are operative to provide arotational interlock between the rotor hub RH and the rotor shaft RSwhile the complementary cone seat CS is operative to provide themechanical support for the rotor hub RH. The aforedescribedconfiguration of the conventional spline/cone Seat arrangement requiresthat the shaft aperture SA be large enough to accommodate the shaftsplines SS and that the diameter D of the rotor shaft RS be sufficientto provide the complementary cone seat CS support surface. The diameterD of the rotor shaft RS is, therefore, a critical dimension thatsignificantly influences the dimensions of the transmission housing andthe swashplate assembly. The conventional spline/cone seat arrangementgenerally results in a heavy rotor assembly having a large radialdimension.

The integrated spline/cone seat subassembly 190 of the present inventionis schematically illustrated in FIG. 9 and in further detail in FIGS.9A, 9B. With reference to FIG. 9, each rotor shaft 128R, 130R is formedto include a primary shaft portion 192 having a first diameter D₁ (thecritical diameter), an end shaft portion 194 having a second diameter D₂where D₁ >D₂, and a truncated conic transition portion 196 intermediatethe portions 192, 194 (see also FIG. 6). The truncated portion 196 makesa predetermined angle β with respect to the rotational axis 101, i.e.,with the axis of the respective rotor shaft. Each end shaft portion 194has a plurality shaft spines 198 extending radially outwardly therefrom.The diameter D₃ defined by the outboard, circumferential surfaces of theshaft splines 198 is equal to the critical diameter D₁ of the primaryshaft portion 192.

Each counter-rotating rotor 200, 202 for the described embodiment of theUAV 10 includes a rotor hub 204 that functions as part of the respectiveintegrated spline/cone seat subassembly 190. Referring to FIGS. 9A, 9B,each rotor hub 204 includes a shaft aperture 206 having a plurality ofhub splines 208 extending radially inwardly from the wall defining theshaft aperture 206. The hub splines 208 and the shaft splines 198 aresized to accommodate the torque required by the counter-rotating rotors200, 202. The specific number and individual thicknesses of the hubsplines 208 complement the specific number and individual thicknesses ofthe shaft splines 198 so that the interleaved hub and shaft splines 208,198 are operative to provide a rotational interlock between each rotorhub 204 and the corresponding rotor shaft 128R, 130R.

The lower portion of each hub spline 208 has an outwardly taperedportion 210 that makes a predetermined angle θ with respect to the hubcenterline 212, i.e., with respect to the rotational axis 101 (see FIG.9). The predetermined angle θ of the outwardly tapered portions 210 ofthe hub splines 208 is equal to the predetermined angle β of thetruncated portion 196. The tapered portions 210 of the hub splines 208,therefore, abuttingly engage and are mechanically supported by thetruncated conic transition portion 196 of the respective rotor shafts128R, 130R. Self-locking nuts 199 (see FIG. 4) are threaded onto theends of the respective rotor shafts 128R, 130R to secure the rotor hubs204 in interlocked, engaged combination with the respective rotor shafts128R, 130R.

The critical diameter D₁ of the primary shaft portion 192 of therespective rotor shafts 128R, 130R is less than the diameter D of arotor shaft that incorporates the conventional spline/cone seatarrangement (compare first diameter D₁ of FIG. 9 with diameter D of FIG.8). The critical diameter D₁ defines the radial dimensions of therespective rotor shafts 128R, 130R, and, in consequence, the sizing ofthe standpipe bearings 132, 134, 136, 138, the multi-member transmissionhousing 140, and the stationary swashplate subassemblies 80.

Each counter-rotating rotor 200, 202 includes the rotor hub 204, foursnubber assemblies 230, and four rotor blade assemblies 250. The rotorhub 204 described in the preceding paragraphs additionally comprisesfour outwardly extending arms 214, each arm 214 having bifurcated ends216U, 216L having bolt holes 218U, 218L, respectively, formedtherethrough as illustrated in FIGS. 9A, 9B. The bifurcated ends 216U,216L and the respective bolt holes 218U, 218L, in combination, form aclevis 220. Each clevis 220, in combination with a respective bolt, nut,washer set 222, is operative to provide the means for securing the rotorblade assembly 250 in combination with the rotor hub 204 as illustratedin FIGS. 10A, 10B, 10C and as described in further detail hereinbelow.

The rotor hub 204 also functions as an element of the snubber assembly230. Each outwardly extending arm 214 of the rotor hub 204 furthercomprises an inboard internal bulkhead 223 and an outboard internalbulkhead 224, which in combination, define a bearing cavity 225, and aninboard cavity 226, as illustrated in FIGS. 9A, 9B. The inboard andoutboard internal bulkeads 223, 224 have bolt holes 227, 228,respectively, formed therethrough. The foregoing elements are operativeto provide the means for securing the respective snubber assembly 230 incombination with the rotor hub 204 as described in the followingparagraphs.

Traditional "bearingless" rotor system designs have the snubber assemblyinstalled outboard of the flexbeam-to-hub attachment joint to reduce thehub length. Such a mounting installation, however, does not facilitateassembly and maintenance of the snubber assembly, and to compensate forsuch a mounting installation, traditional rotor systems incorporateexpensive elastomeric bearings that are more wear resistant thaninexpensive self-aligning bearings to minimize maintenance requirements.To utilize an outboard mounting installation in a UAV of the typedescribed herein, the flexbeam of each rotor blade assembly 250(described in further detail hereinbelow) would require a slottedconfiguration so that the respective snubber assembly passes through theflexbeam for securement to the upper and lower surfaces of therespective integrated torque tube/spar member. Since this inboardsegment of the flexbeam is a highly loaded zone, the flexbeamconfiguration would have to be widened, which would require wider rotorhub arms, to accommodate the high loading. In addition, theflexbeam-to-hub bolts would have to be located closer to the center ofthe rotor hub, and would have to be larger to accommodate thecorresponding high loading. These features would increase the overallweight of the rotor assembly for the UAV.

The rotor hub 204 configuration described hereinabove facilitatesinstallation of the respective snubber assembly 230 of the presentinvention inboard of the flexbeam-to-hub attachment joint as illustratedin FIGS. 10A, 10B, 10C. The inboard installation eliminates the need forany structural modifications of the flexbeam, minimizes the widthrequirements of the hub arms 214, and allows the use of a self-aligningbearing that is less expensive than an elastomeric bearing. The inboardinstalled snubber assembly 230 is also more accessible for assembly andmaintenance, resulting in reduced labor costs for such activities.

The snubber assembly 230 of the present invention is illustrated infurther detail in FIGS. 10C, 10D and comprises a spherical bearing 232,a bearing bolt 234, a locking nut 236, a snubber bracket 238 secured incombination with the spherical bearing 232, and securing bolts 240. Thespherical bearing 232, snubber bracket 238 combination is rotatablymounted within the bearing cavity 225 by means of the bearing bolt 234which is inserted through the bolt hole 227, the spherical bearing 232,and the bolt hole 228, respectively. The bearing bolt 234 is secured incombination in the rotor hub 204 by means of the locking nut 236 whichis threaded onto the bearing bolt 234 to jam against the inboardbulkhead 223. The securing bolts 240 are utilized to secure theintegrated torque tube/spar member 270 in combination with the snubberassembly 230 as described hereinbelow in further detail.

Each counter-rotating rotor 200, 202 includes four rotor bladeassemblies 250. Each rotor blade assembly 250 comprises an innerflexbeam 260, an integrated torque tube/spar member 270, an outeraerodynamic fairing or rotor blade 280, and a blade joint 290, asillustrated in FIGS. 10A, 10B. Each rotor blade assembly 250 has atapered configuration that provides reduced weight, low inertia, a highchord frequency, an improved aerodynamic profile, low static droop, andeliminates high chordwise stresses and the need for blade dampingmechanisms.

The flexbeam 260 of the rotor blade assembly 250 is a laminatedcomposite structure that is operative to react the centrifugal loads anda majority of the bending loads developed during operation of thecounter-rotating rotors 200, 202. The inboard end 262 of the flexbeam260 is inserted into the clevis 220 and fastened in combinationtherewith by means of the bolt, washer, nut sets 222 (two in theillustrated embodiment) to secure the flexbeam 260 in combination withthe rotor hub 204 as illustrated in FIGS. 10A, 10B, 10C. The outboardend 264 of the flexbeam 260 is secured in combination with therespective integrated torque tube/spar member 270 and tapered rotorblade 280 by means of the blade joint 290, as described in furtherdetail hereinbelow.

One aspect of rotor blade design involves accounting for the spanwisevariation of the resultant velocity vector, which is a combination ofthe rotational velocity vector acting on the rotor blade and the airinflow velocity vector perpendicular to the rotor plane, acting on therotor blade. The spanwise variation of the resultant velocity vectorresults in a variation in the downwash angle, i.e., the angle betweenthe resultant velocity vector and the rotor plane, along the span oftapered rotor blade 280. If the rotor blade has a constant pitch anglealong the span, the angle of attack, i.e., the angle between theresultant velocity vector and the airfoil chord, will be less thanoptimum, resulting in poor rotor blade performance.

To produce a near optimum angle of attack distribution along the span ofthe tapered rotor blade 280, rotor blade airfoil sections are normallypretwisted. Pretwisted rotor blades are operative to pitch in flight asrigid bodies, i.e., uniformly along the span, in response to controlcommands to adjust for variations in flight conditions. The spanwiseuniform pitch angle can be either constant with respect to blade azimuthposition (collective pitch) or sinusoidally variable with respect toblade azimuth position (cyclic pitch).

Whatever the final pitch position of the rotor blade at the outboardjoint with the flexbeam, the flexbeam must be twisted to the same angleso that it fits inside the blade spar to form a clean, minimum thicknessoutside airfoil. If twisting of the flexbeam is all elastic, very hightwisting strains are induced in the flexbeam. Generally, the outboardend of the flexbeam is locally pretwisted to accommodate the bladecollective pitch required during normal flight modes. For the UAVdescribed herein, however, local pretwisting of only the outboard end ofthe flexbeam would create high kick loads in the flexbeam laminate,resulting in possible delaminations between plies during operation ofthe rotor assembly 100.

To compensate for the variable strains induced along the span of theflexbeam, the flexbeam 260 of the present invention is fabricated tohave a predetermined linear twist, i.e., built-in twist, along the spanthereof (inboard end 262 to outboard end 264). As a result of suchpretwisting (which is not explicitly shown in FIGS. 10A, 10B, 10C forpurposes of clarity), the pretwisted flexbeam 260 makes an angle withrespect to a horizontal plane HP (see FIG. 10B) that varies linearlyfrom about 0° at the inboard end 262 (root section) of the pretwistedflexbeam 260 to about 22° at the outboard end 264 (tip section) of thepretwisted flexbeam 260. The linear pretwist of the flexbeam 260 of thepresent invention is defined in the graph of FIG. 13.

The angle of the pretwisted flexbeam 260 corresponds to the elastictwist that an untwisted flexbeam would normally experience duringforward flight of the UAV 10, i.e., rotor blade tip speed of about 700fps under normal thrust during cruise conditions. As a result, thepretwisted flexbeam 260 is unstrained during such forward flightconditions. The pretwist of the flexbeam 260 also minimizes the flexbeamelastic twist required to accommodate the pitch motion of the rotorblade 280 in all other normal flight modes, i.e., the induced strainsare reduced due to the linear pretwist of the flexbeam 260 according tothe present invention.

The integrated torque tube/spar member 270 of the rotor blade assembly250 is formed as a continuous, single piece, low cost tubular compositestructure. The structural configuration of the integrated torquetube/spar member 270 provides high torsional and bending stiffness andfacilitates the utilization of the efficient blade joint 290 describedin further detail hereinbelow. Conventional rotor blade design, incontrast, generally involves the combination of several structural andnon-structural elements to form the blade subassembly. For example,conventional blade subassemblies usually include separate members forreacting dynamic torsional and bending loads. This design philosophy isdeficient inasmuch as it requires separate members to react dynamicloads, and it results in inefficient joints.

The integrated torque tube/spar member 270 includes an inboard torquetube segment 272 and a outboard spar segment 274, as illustrated in FIG.10A. The integrated torque tube/spar structure 270 is formed as acontinuous filament wound piece that provides a continuous torsion loadpath and facilitates load coupling from the tapered rotor blades 280into the respective flexbeams 260. The spar segment 274, which functionsas the primary structural member of the rotor blade assembly 250, has atruncated aerodynamic profile as illustrated in FIG. 12A and isoperative to react all bending, torsional, shear, and centrifugaldynamic loads developed during operation of the counter-rotating rotors200, 202. The torque tube segment 272 has a generally elliptical profileas illustrated in FIG. 12B and is operative to react all torsional loadsand some of the bending loads developed during operation of thecounter-rotating rotors 200, 202. The inboard end of the torque tubesegment 272 is secured in combination with the snubber bracket 238 ofthe snubber assembly 230 by means of the securing bolts 240 (see FIG.10C) which extend through the inboard wall of the torque tube segment274. Pitch inputs from the swashplate subassemblies 80 are coupled intothe rotor blade assemblies 250 of the counter-rotating rotors 200, 202by means of the respective torque tube segments 272 which are twistableabout the bearing bolt 234 of the snubber assembly 230 (FIG. 10Dillustrates a torque tube segment 272 in a twisted condition, i.e.,pitch input applied thereto).

Rotor systems for conventional rotorcraft are designed to provide anautorotation capability. To facilitate operation of the rotor system inthe autorotation mode, conventional rotor blades generally incorporateancillary mass near the blade tips to increase blade inertia. Therelatively high inertia of such rotor blades presents a problem at startup inasmuch as greater engine torque must be provided to initiate rotorblade rotation. The high blade inertia of conventional rotor systemsalso creates an additional problem inasmuch as such systems result inchordwise natural frequencies near the 1/rev resonance frequency, whichis the highest amplitude excitation frequency (see FIG. 11). Operationof a rotor system near the 1/rev resonance mode is generally undesirabledue to high induced loading, and in consequence, operation ofconventional stiff, in-plane rotor systems is generally constrained torotor speeds that fall between resonance mode conditions, as exemplarilyrepresented by operating curve CRS in FIG. 11.

The UAV 10 of the present invention does not require an autorotationcapability. In consequence, the configuration of the rotor blade 280 ofeach rotor blade assembly 250 may be design optimized for reduced weightutilizing composite materials, e.g., high modulus graphite, whichresults in a tapered rotor blade 280 having a high chord frequency. Eachoutboard segment of the rotor blades 280 of the present invention isconfigured to have an aerodynamic taper of about 2:1, which results in atapered rotor blade 280 having a low outboard mass and a high inboardstiffness. With reference to FIG. 10A where reference numeral 282defines the aerodynamic root of the rotor blade 280 and referencenumeral 284 defines the aerodynamic tip of the rotor blade 280,aerodynamic taper is defined as the ratio of the effective chord at theaerodynamic root 282 to the effective chord at the aerodynamic tip 284.

The aerodynamic taper of the outer rotor blades 280 results in a lowmoment of inertia about the hub centerline 212 and the mass centroid ofeach rotor blade 280 being closer to the rotor hub 204. The highchordwise frequency of the tapered rotor blades 280 provides the benefitof rotor operation over a weaker modal response zone, i.e., the highfrequency design provided by the tapered rotor blades 280 eliminatesoperation in critical resonance mode conditions which may occur due tothe wide RPM operating range of the UAV 10 of the present invention.Referring to FIG. 11, reference numeral 286 identifies the operatingcurve of a UAV 10 incorporating rotor blade assemblies 250 having thetapered rotor blade 280 configuration described hereinabove.

For the normal operating range of the UAV 10, i.e., between a 550 fpshover mode and a 700 fps cruise mode (wherein the fps values reflectblade tip speeds), the counter-rotating rotors 200, 202 operate betweenthe 2/rev and 3/rev resonance mode conditions, i.e., generally above2.5/rev. These resonance mode conditions are lower load conditions incomparison to the 1/rev resonance mode condition, which is the highestload condition (see FIG. 11), such that induced loading of the rotorblade assemblies 250 is reduced. In addition, the high frequency designof the rotor blade assembly 250 eliminates ground and air resonanceduring UAV 10 operations, and thus eliminates requirements for lagdampers.

Each tapered rotor blade 280 is further configured to include a trailingedge segment 286 of generally triangular shape (see FIGS. 12A, 12B). Thetrailing edge segment 286 of the rotor blade 280 is a continuousstructural member that extends aftwardly from the integrated torquetube/spar member 270 as illustrated in FIGS. 10A, 12A, 12B. Theconfiguration of the trailing edge segment 286 provides a low weightblade configuration that is design optimized for the aerodynamicpressures encountered during operation of the "shrouded"counter-rotating rotors 200, 202.

The blade joint 290 of the rotor blade assembly 250 according to thepresent invention is illustrated generally in FIGS. 10A, 10B and infurther detail in FIG. 14. As disclosed hereinabove, the blade joint 290is operative to secure the pretwisted flexbeam 260 in combination withthe respective integrated torque tube/spar member 270 and tapered rotorblade 280. The blade joint 290 of the present invention includes aninnovative bolt layout that is optimally positioned to eliminate themoment reaction at the blade joint 290 due to the steady chordwiseloading experienced by the rotor blade assembly 250, thereby allowingthe utilization of smaller bolts 298 and reduced joint thicknesses.

A conventional rotor blade attachment joint is designed and positionedto react the axial loading resulting from the centrifugal force exertedon the rotor blade during operation thereof. A rotor blade assembly hasan identifiable centroid for that portion of the rotor blade assemblymass outboard of the blade attachment joint. Such outboard mass centroidlies on the centroidal axis of the rotor blade assembly. The centrifugalforce vector acting on the rotor blade acts through the outboard masscentroid. The conventional attachment joint is designed and positionedsuch that the center of the attachment joint lies near the centroidalaxis such that the centrifugal force vector acts through the attachmentjoint center, i.e., the attachment joint is subjected to only axialloading due to the centrifugal force vector, there is no in-planereaction moment at the attachment joint due to the centrifugal forcevector.

The conventional attachment joint is further designed to react anin-plane moment arising from the chordwise aerodynamic and inertia loadsexperienced by the rotor blade assembly during operation thereof. Such achordwise load vector causes a reaction moment at the attachment jointcenter. The effect of the chordwise bending moment is to induce largesteady stresses in the attachment joint, which reduces the fatigueallowable therein. To compensate for the foregoing effects, aconventional rotor blade attachment joint utilizes heavy bolts andenhanced joint thicknesses.

Referring to FIG. 14, each rotor blade assembly 250 of the presentinvention has an identifiable outboard mass centroid 292 that lies onthe blade centroidal axis 294. The centrifugal force vector CFV actsthrough the outboard mass centroid 292 as shown. The steady chordwiseload vector CLV, which includes aerodynamic as well as inertia loads,acts generally perpendicular to the centroidal axis 294 as illustrated.Since the chordwise load vector CLV includes an aerodynamic loadcomponent, the chordwise load vector CLV of the rotor blade assembly 250does not act through the outboard mass centroid 292.

The blade joint 290 includes a bolt pattern 296 comprised of a pluralityof bolts 298 (four in the illustrated embodiment) arranged to define astructural center 300 for the bolt pattern 296. Extending through thebolt pattern structural center 300, parallel to the blade centroidalaxis 294, is a bolt pattern center axis 302.

According to the present invention, the bolt pattern 296 of the bladejoint 290 is disposed in combination with the rotor blade assembly 250so that the bolt pattern center axis 302 is spaced apart a predetermineddistance D from the blade centroidal axis 294. Further, the blade joint290 is disposed in combination with the rotor blade assembly 250 so thatthe bolt pattern structural center 300 is spaced apart a predetermineddistance X from the steady chordwise load vector CLV (one skilled inaerodynamics, based upon the configuration of the rotor blade assembly250 and nominal cruise condition of the UAV 10, can compute thepredetermined distance X). The location of the bolt pattern 296 withrespect to the outboard mass centroid 292 and the chordwise load vectorCLV, as defined by the predetermined distances D, X, respectively, issuch that the reaction moments M_(CLV), M_(CFV) at the blade joint 290due to the chordwise load vector CLV and the centrifugal force vectorCFV, respectively, cancel out, i.e., M_(CLV) =M_(CFV), at the nominalcruise condition of the UAV 10, e.g., blade tip speed of about 700 fps.An examination of FIG. 14 shows that

    (CFV)×(D)=(CLV)×(X)

since M_(CLV) =M_(CFV). FIG. 15 is a graphical depiction of thepredetermined distance D in terms of the moments produced by thecentrifugal force vector CFV and the chordwise load vector CLV.

A further examination of FIG. 14 shows that the structural center 300 ofthe bolt pattern 296 lies forward of the outboard mass centroid 292,i.e., forward of the centroidal axis 292. The functional resultsdescribed hereinabove could not be achieved by moving the outboard masscentroid 292 aft since the forward position of the outboard masscentroid 292 is required for stability of the rotor blade assembly 250.Positioning of the blade joint 290 as described hereinabove does notaffect stability and will only result in a local moment change. Momentsat other span locations will remain unaffected.

The blade joint 290 described hereinabove was a bolt pattern 296 formedby four bolts 298. Those skilled in the art will appreciate that otherbolt patterns, i.e., more or less than four bolts, may be utilized forthe blade joint 290 of the present invention. Any such other boltpattern must define a bolt pattern structural center and a bolt patterncenter axis that provides the predetermined distances D, X describedhereinabove.

Conventional rotor assemblies include a plurality of pitch control rodssecured in combination with respective torque tubes of the rotor bladesubassemblies. The pitch control rods are operative to couple collectiveand/or cyclic pitch inputs to individual rotor blades via the respectivetorque tubes. A conventional mounting scheme for pitch control rods isillustrated in FIG. 16. A control rod bearing CRB is mounted to an endof the pitch control rod (not shown). The control rod bearing CRB ismounted within a bearing mount BM, by means of a bearing attachment boltBAB, that is rigidly secured to the torque tube TT of the respectiverotor blade subassembly. The conventional mounting scheme results in theweak axis WA of the control rod bearing CRB facing outboard, i.e.,approximately parallel to the rotor blade centerline CL.

The conventional mounting scheme described in the preceding paragraph isnecessary so that the pitch control rods have the required range ofmotion to impart the full range of pitch inputs to the rotor blades.This mounting scheme is deficient in that the high centrifugal loadF_(CL) of the pitch control rod acts in the direction where the controlrod bearing CRB is weakest. This causes high pressure forces HPF to beexerted against the bearing liner, which causes the control rod bearingCRB to wear out rather quickly. The replacement frequency for suchcontrol rod bearings CRB, due to the effects of centrifugal loading,results in increased maintenance costs and system downtime.

The rotor assembly 100 of the present invention does not require thefull range of pitch inputs required by conventional rotor assemblies. Asa result, the range of motion for the pitch control rods 104 (see FIG.4) is less than the range of motion required by conventional rotorassemblies. In consequence, the rotor assembly 100 of the presentinvention utilizes an optimized pitch control rod mounting scheme 310 asillustrated generally in FIG. 4 and in further detail in FIG. 17.

A control rod bearing 312 is mounted to the end of a respective pitchcontrol rod 104 (see FIG. 4). The control rod bearing 312 is mountedwithin a bearing mount 314 having a clevis configuration by means of abearing attachment bolt 316. The bearing mount is rigidly secured to thetorque tube segment 272 of the respective rotor blade assembly 250. Thebearing mount 314 is configured so that the axis 316A of the bearingattachment bolt 316 makes an angle β with respect to the centerline 318of the rotor blade assembly 250. The angle β corresponds to thedirection of centrifugal loading F_(CL) such that the pressure forces320 acting on the control rod bearing 312 as a result of centrifugalloading F_(CL) are exerted against the strongest part of the control rodbearing 312.

The optimized pitch control rod mounting scheme 310 describedhereinabove results in the pressure forces 320 acting on the strongestpart of the control rod bearing 312. As a result, the optimized pitchcontrol rod mounting scheme 310 results in longer effective lifetimesfor the control rod bearings 312.

A variety of modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the presentinvention may be practiced otherwise than as specifically describedhereinabove.

What is claimed is:
 1. A coaxial transmission/center hub subassembly fora rotor assembly of an aerial vehicle having ducted counter-rotatingrotors defining a rotational axis for the aerial vehicle, the aerialvehicle having a toroidal fuselage with a plurality of inwardlyextending radial struts and including an engine, the rotor assemblyincluding upper and lower swashplate subassemblies, comprising:centerhub support structure means for mounting said coaxialtransmission/center hub assembly in fixed coaxial relation to thetoroidal fuselage utilizing the plurality of inwardly extending radialsupport struts; a multi-member transmission housing includingan upperstandpipe housing secured in combination with said center hub supportstructure means, and a lower standpipe housing secured in combinationwith said center hub support structure means; and a single stagetransmission subsystem includingan upper bevel gear including an upperrotor shaft rotatably mounted in combination in said upper standpipehousing, a lower bevel gear including a lower rotor shaft rotatablymounted in combination in said lower standpipe housing, said upper andlower rotor shafts providing means for mounting the counter-rotatingrotors in combination with said coaxial transmission/center hubsubassembly, and an input gear rotatably mounted in combination in saidcenter hub support structures means, said input gear being mechanicallycoupled in combination with said upper and lower bevel gears and theengine to couple torque therefrom to the counter-rotating rotors; andwherein said upper and lower standpipe housings are secured incombination with said center hub support structure means in opposedrelation along the rotational axis of the aerial vehicle; and whereinexternal surfaces of said upper and lower standpipe housings providesliding surfaces for bi-directional linear movement of the upper andlower swashplate subassemblies, respectively.
 2. The coaxialtransmission/center hub subassembly of claim 1 wherein said center hubsupport structure means includes a cylindrical body having at leastthree integral, outwardly extending radial support arms for rigidlyattaching said coaxial transmission/center hub subassembly incombination with the plurality of inwardly extending radial support armsin fixed coaxial relation with the toroid fuselage.
 3. The coaxialtransmission/center hub subassembly of claim 2 wherein said center hubsupport structure means further comprises a middle housing secured incombination with said cylindrical body so that external surfaces of saidmiddle housing abuttingly engage internal surfaces of said center hubsupport structure, said middle housing forming part of said multi-membertransmission housing wherein said upper and lower standpipe housings aresecured in combination therewith and said input gear is rotatablymounted in combination therein.
 4. The coaxial transmission/center hubsubassembly of claim 1 wherein said tempter hub support structure meanscomprises:a center hub support structure including a cylindrical bodyhaving at least three integral, outwardly extending radial support armsfor rigidly attaching said coaxial transmission/center hub subassemblyin combination with the plurality of inwardly extending radial supportstruts in fixed coaxial relation with the toroidal fuselage; and amiddle housing secured in combination with said cylindrical body so thatexternal surfaces of said middle housing abuttingly engage internalsurfaces of said center hub support structure; said middle housingforming part of said multi-member transmission housing wherein saidupper and lower standpipe housings are secured in combination therewithand said input gear is rotatably mounted in combination therein.
 5. Thecoaxial transmission/center hub subassembly of claim 4 wherein saidmulti-member transmission housing is internally configured to provide asplash lubrication subsystem for said single stage transmissionsubsystem.
 6. The coaxial transmission/center hub subassembly of claim 4wherein said multi-member transmission housing includes a splashlubrication subsystem comprising:said middle housing including a pinionchamber; said upper and lower standpipe housings having standpipechambers and cored passages; and said upper and lower standpipe housingsin combination defining a central reservoir for oil for said splashlubrication subsystem; said central reservoir being fluidicallyinterconnected to said pinion chamber, said standpipe chambers, and saidcored passages to define said splash lubrication subsystem; rotarymotion of said upper and lower bevel gears circulating oil through saidsplash lubrication subsystem.
 7. The coaxial transmission/center hubsubassembly of claim 1 wherein said center hub support structure meanscomprises:a center hub support structure including a cylindrical bodyhaving at least three integral, outwardly extending radial support armsfor rigidly attaching said coaxial transmission/center hub subassemblyin combination with the plurality of inwardly extending radial supportstruts in fixed coaxial relation with the toroidal fuselage; and amiddle housing fabricated as an integral part of said cylindrical body;said middle housing forming part of said multi-member transmissionhousing wherein said upper and lower standpipe housings are secured incombination therewith and said input gear is rotatably mounted incombination therein.
 8. The coaxial transmission/center hub subassemblyof claim 7 wherein said multi-member transmission housing includes asplash lubrication subsystem comprising:said middle housing including apinion chamber; said upper and lower standpipe housings having standpipechambers and cored passages; and said upper and lower standpipe housingsin combination defining a central reservoir for oil for said splashlubrication subsystem; said central reservoir being fluidicallyinterconnected to said pinion chamber, said standpipe chambers, and saidcored passages to define said splash lubrication subsystem; rotarymotion of said upper and lower bevel gears circulating oil through saidsplash lubrication subsystem.
 9. A coaxial transmission/center hubsubassembly for a rotor assembly of an aerial vehicle having ductedcounter-rotating rotors defining a rotational axis for the aerialvehicle, the aerial vehicle having a toroidal fuselage with a pluralityof inwardly extending radial support struts and including an engine, therotor assembly including upper and lower swashplate subassemblies,comprising:a center hub support structure that includes a cylindricalbody having at least three integral, outwardly extending radial supportarms for rigidly attaching said coaxial transmission/center hubsubassembly in combination with the plurality of inwardly extendingradial support struts in fixed coaxial relation with the toroidalfuselage; a multi-member transmission housing includinga middle holdingsecured in combination with said cylindrical body so that externalsurfaces of said middle housing abuttingly engage internal surfaces ofsaid center hub support structure, an upper standpipe housing secured incombination with said middle housing, and a lower standpipe housingsecured in combination with said middle housing, and a single stagetransmission subsystem includingan upper bevel gear including an upperrotor shaft rotatably mounted in combination in said upper standpipehousing, a lower bevel gear including a lower rotor shaft rotatablymounted in combination in said lower standpipe housing, said upper andlower rotor shafts providing means for mounting the counter-rotatingrotors in combination with said coaxial transmission/center hubsubassembly, and an input gear rotatably mounted in combination in saidmiddle housing, said input gear being mechanically coupled incombination with said upper and lower bevel gears and the engine tocouple torque therefrom to the counter-rotating rotors; and wherein saidupper and lower standpipe housings are secured in combination with saidmiddle housing in opposed relation along the rotational axis of theaerial vehicle; and wherein external surfaces of said upper and lowerstandpipe housings provide sliding surfaces for bi-directional linearmovement of the upper and lower swashplate subassemblies, respectively.